The present invention relates to silicon-containing polymers and methods for the preparation and use thereof. In particular, the present invention is directed to novel high-molecular weight silicon-containing polymers which may exhibit high degrees of branching and/or cross-linking, as well as to novel monomers and methods for use in the preparation thereof.
Polysilanes and other silicon-containing polymers have been the focus of considerable interest, in particular because of their unusual optical and electronic properties. An interesting class of silicon-containing polymers are the poly[(disilanylene)-arylene]s, for which orbital interactions of the aromatic .pi.-system with the adjacent disilanylene units are possible. The heretofore known compounds have the general formula: ##STR1## in which R and R' are independently selected from alkyl and aryl, Ar is an arylene and n is relatively low.
For example, Nate et al. have described the preparation of p-(disilanylene)-phenylene polymers by reaction of 1,4-di(chloroethylmethyl-silyl)benzene or 1,4-di(chloromethylphenyl) benzene with sodium metal in toluene [Nate, K. et al., Electrochem. Soc. Extended Abstr. 94(2):778 (1984)]. The polymers were reported to have strong absorption in the ultraviolet region and considered to be able to act as positive working photoresists. After exposing these polymer films through a photomask, a positive resist pattern with fine line resolution was obtained, using a mixture of toluene-isopropylalcohol as developer and isopropylalcohol as rinser. The photosensitivity of the resist film was enhanced by addition of radical trapping agents. Poly[(disilanylene)-phenylene] polymers prepared by Wurtz-type coupling reactions have been reported with molecular weights of ca. 20,000-35,000. [Nate, K. et al., Organometallics 6:1673 (1987)].
The Lewis acid-catalyzed ring opening polymerization of 1,1,2,2-tetramethyl-1,2-disilabenzocyclobutene in a sealed tube has been reported to yield poly[o-(disilanylene)phenylene]s with higher molecular weights (M.sub.n =56,000-350,000); in view of ring strain in the monomer, the polymerization is reported to occur spontaneously [Shiina, K., J. Organomet. Chem 310. C57 (1986)]. The high-molecular weight, cotton wool-like material was soluble in solvents such as benzene, THF, cyclohexane, carbon tetrachloride and chloroform.
More recently, (disilanylene)phenylene polymers containing naphthalene [Lee, S.-J. H. and Weber, W. P., Polymer Bulletin 22, 355 (1989)], thiophene [Hu, S.-S. and Weber, W. P., Polymer Bulletin 22, 355 (1989)], and furan [Hong, H. H. and Weber, W. P., Polymer Bulletin 22, 363 (1989)] arylene subunits have been obtained via reductive Wurtz coupling methods. All three of these polymers show a shift of .lambda..sub.max to longer wavelength compared to the corresponding monomers, suggesting some degree of .sigma.-.pi. conjugation. Photolysis of these compounds in benzene/methanol solution led to their rapid degradation.
Photodegradation studies of (disilanylene)phenylene polymers indicates their utility as possible photoresist materials. In addition, organic silicon polymers having high moldability and processability can be shaped, as preceramic materials, into suitable forms such as fibers or sheets; the polymers, alone or in admixture with other ceramic precursor materials and/or metals, may then be sintered to obtain final ceramic or cermet products. Some silicon-containing polymers further have utility as binders, coating materials, coupling agents, adhesives and adhesion promoters. As disclosed in U.S. Pat. No. 3,278,461 to Wu, the entire disclosure of which is hereby incorporated by reference, polymers containing silicon-silicon bonds are particularly desirable, in that the silicon-silicon bond acts as a "scavenger" of oxygen, so that at elevated temperatures, particularly in oxidizing atmospheres, the material containing silicon-silicon bonds is not destroyed by oxygen; the oxygen merely reacts with the silicon-silicon bond under such circumstances to form silicon-oxygen linkages.
Unfortunately, preparative methods useful for synthesis of these polymers on a commercial scale had not been developed. Moreover, the known methods have been limited to the use of monomers having tertiary silicon centers (i.e., monomers of the general formula XSiRR'-Ar-R'RSiX, wherein neither R nor R' may be hydrogen).
Methods for the preparation of polysilanes, on the other hand, have been developed recently to make commercial-scale synthesis of those materials economically feasible. In particular, Zr- and Hf-catalyzed dehydrocoupling reactions of hydrosilanes (Cp=.eta..sup.5 -C.sub.5 H.sub.5, Cp*=.eta..sup.5 -C.sub.5 Me.sub.5) have been developed and characterized as metal-mediated step-growth polymerizations [Woo, H.-G. and Tilley, T. D., J. Am. Chem. Soc. 111:8043 (1989). Reactions of this type, originally described by Harrod and coworkers, appear to represent an important new method for forming Si-Si bonds and generating Si-containing polymers [Aitken, C. et al., J. Organomet. Chem. 279:C11 (1985); Aitken, C.T. et al., J. Am. Chem. Soc. 108:4059 (1986)]. This method, however, has heretofore been applicable to produce polymers of only modest chain lengths (generally .ltoreq. ca. 50 silicon atoms) from monosilane monomers.
It is an object of the present invention to provide silicon-containing polymers of higher molecular weight than have heretofore been produced using the known synthetic methods.
It is a further object of the invention to provide methods for the preparation of silicon-containing polymers in which there is a significant degree of branching and/or cross-linking between silicon centers.
It is yet another object of the present invention to provide silicon-containing polymers derived from polyfunctional monomers comprising primary and secondary silicon centers.
It is a further object of the invention to provide high molecular weight, processable silicon-containing polymers for use in the preparation of films, fibers and the like.
It is an additional object of the present invention to provide novel monomers for use in the preparation of silicon-containing polymers.